<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body {
            height: 2000px;
        }
        
        .back {
            position: fixed;
            width: 50px;
            height: 50px;
            background-color: pink;
            right: 30px;
            bottom: 100px;
            display: none;
        }
        
        .container {
            width: 900px;
            height: 500px;
            background-color: skyblue;
            margin: 400px auto;
        }
    </style>
    <script src="js/jQuery.min.js"></script>
</head>
<body>
    <div class="back">返回顶部</div>
    <div class="container">
    </div>
    <script>
        $(function(){
            $(document).scrollTop(100)
            var boxTop = $('.container').offset().top;
            $(window).scroll(function(){
                console.log($(document).scrollTop());
                if($(document).scrollTop() >= boxTop){
                    $('.back').fadeIn();
                }else {
                    $('.back').fadeOut();
                }
            })
            // 返回顶部
            $('.back').click(function(){
                $('body,html').stop().animate({
                    scrollTop:0
                })
                //不能是文档，只能是html body元素 做动画
            })
        })
    </script>
</body>
</html>